The invention relates generally to the area of numerically controlled machine tools: and specifically, the invention provides a method and apparatus for controlling the operation of a machine tool as a function of the cutting torque load on a rotating spindle. There are a great number of machine tool control systems in the prior art. Of particular interest in this application are systems in which torque is the measured control variable. However, the principle claimed herein is applicable to many other systems.
Torque is typically a viable control variable in the drilling process. There are three major variations in the drilling process, and the merit of a control system lies in its ability to adjust for these variations. First, there are changes in the material being drilled. This is caused by changes in material hardness and changes in machineability. Second, the control system must be capable of accomodating changes in the drill. Torque changes will occur because of chip clogging in the drill flutes, and torque will change as a function of the relative sharpness of the drills. Third, the control system must be capable of controlling the machine operation when the drill breaks through from the workpiece into air. Not only does this occur when drilling throughholes, but also when encountering cross holes within the workpiece. The torque control system must sense the rise in torque caused by any one or more of the above variables and decrease the feed rate so as to maintain the torque within the desired limits. When utilizing a machine without a torque control, the operator has to ensure that the torque does not build up excessively in the worst case. Therefore, the machine must be operated in a conservative manner. Since the torque control adjusts the feed rate to changing cutting conditions, the machine can generally be operated at a higher average feedrate.
It is a well-known fact that torque may be measured by sensing spindle motor current and voltage to determine the power supplied by the spindle motor and further sensing the angular velocity of the cutting tool to determine the torque. A number of torque sensitive controls have been commercially available which utilize the above general relationship to determine torque and modify the machine operation as a function thereof. Typically, traditional systems are fully analogue which results in several inherent disadvantages.
The most significant disadvantage is that a single linear scale must be used to define the full range of the measured torque variable. A typical linear range for analog devices is 10 volts. A typical range of spindle motor current is 100 milliamps to 120 amps., i.e., a range of 4 orders of magnitude or decades. Consequently, the analog transducers must have millivolt sensitivity without any loss of resolution--an expensive requirement to satisfy.
The problem is compounded further by the necessity of multiplying the current and voltage variables to obtain power. For the magnitudes previously mentioned, the analogue multiplier must have a range of seven orders of magnitude or decades. This is a practical impossibility for commercially available components which can be economically used. It is readily obvious that the dynamic range of a linear system must be limited to under three orders of magnitude or decades, and therefore such a system has severe inherent limitations in the present application.
To improve the capability and versatility of the control system, a microprocessor based system is desirable; however, new problems are introduced which are not found in the analogue system. These problems relate to diagnostic procedures, communication with other elements in the system and the determination of torque itself. Each torque measurement requires that the variables defining torque be measured, the value of torque be calculated, the amount of excessive torque be determined and a remedial action be taken in time to protect the tool.
In contrast to an analogue system in which the variables may be measured at any time or on a continuous basis; in a digital system, the variables can only be sampled at a particular instant in time. This poses a difficult problem in determining the torque at idle or in a no-load condition. The idle torque can vary over a two-to-one range during a single spindle revolution; therefore, a single sample of the variables is inadequate. Consequently, the variables defining torque must be sampled several times during a single spindle revolution. From these idle torque samples a reasonable average idle torque can be determined and stored.
The general problem of determining torque and the particular problem of detecting the idle torque is further complicated by the necessary range of operation of the system. For example, a typical maximum spindle speed is 6000 revolutions per minute; and to achieve a reasonable number of idle torque samples, e.g. 10 samples per revolution, the system must be capable of making a torque determination every millisecond. In one millisecond, the system must measure the spindle motor current, voltage and speed; multiply the current times the voltage; divide the product by the speed and store the quotient. Therefore, even though the decision to use a microprocessor based system may conceptually eliminate the limitations of the analogue based systems, new problems relating to torque detection and determination must be solved.
The disclosed invention provides a torque control system capable of measuring torque over five orders of magnitude which provides the necessary idle sampling values and which allows more flexibility in the area of machine control.